Trilaminar germ disc
By
Dr Manah Chandra Changmai MBBS MS
Gastrulation
Formation of embryonic mesoderm and endoderm
Primitive streak on the surface of the epiblastThe cephalic end of the streak,known as primitive nodePrimitive node surrounds the a small primitive pit
Day 14 -15 raised groove appearson dorsal surface of the epiblast –cells migrate inward at the streak,displacing underlying hypoblast tobecome the endoderm
Day 16 – ingressing epiblast cellsform the mesoderm (a new layerbetween the epiblast andendoderm)
Epiblast cells that remain on theembryo’s dorsal surface make upthe ectoderm
Ectoderm & endoderm = epitheliaMesoderm = mesenchyme tissue(mesen=middle; chyme=fluid)
Primitive streak
Cells invaginating the primitive pit Move straight forward in cephalic direction upto prochordal plate.
Form a tube like process known as notochordal or head process.
By the 18th day,floor of the notochordal process fuses withunderlying endoderm
Lumen of the notochordalprocess disappears completely
Notocordal process forms anarrow plate of cells
Notochordal cells proliferateand forms solid cord,known asdefinitive notochord.
Formation of notochord
Formation of cloacal membrane
Cloacal membrane is formed caudal to primitive streak,consists of tightly adherent ectoderm and endoderm.
At 16th day a diverticulum is formed at the posterior wallof yolk sac which extends into connectingstalk.
This diverticulum is called allantoenteric diverticulum orallantois.
In humans it remains rudimentary.
Embryonic disc,initially flat and almost round
Later becomes elongated with broad cephalicand narrow caudal end.
At the end of 4th week primitive streak showsregressive changes and soon disappears.
Sometimes remnants of primitive streak persistspersists and at birth cause tumours of sacrococcygealregion.
Changes in embryonic disc
Further development of tropoblast
After 3rd week mesodermal cellspenetrate the core of primary villiforming secondary villi.
At the end of 3rd week mesodermal cellsin the core of villi differentiate into bloodcells forming tertiary villus.
Tertiary villi
Cytotropoblastic cell penetrate into the syncytium until they reach thematernal endometrium
Establish contact with similar neighbouringvillous stems forming thin outer cytotropoblasticshell.
Villi that extend from chorionic plate form deciduabasalis called the stem or anchoring villi.
Chorionic cavity becomes larger by 19th or 20th
day
Embryo is attached to tropoblastic shell by connecting stalk
Further development of tropoblast.
The end